Severe hemorrhage is associated with increased intracellular accumulation of sodium and water, reflecting membrane depolarization i.e., decreased activity of the sodium pump. Since others have described normal or increased ATP during this depolarization, they hypothesize that Na/K ATPase is inhibited in shock. Because this defect is seen in red blood cells, a circulating factor appears to be involved. This factor has been identified as the 33 amino acid C-terminal fragment of Plasminogen Activator Inhibitor type-1 (PAI-1), generated by the cleavage of PAI-1 at its Arg-Met active center by plasminogen activators. Synthetic 33aa fragment causes dose-dependent inhibition of Na/K ATPase in membrane vesicles. The fragment is called Sodium Pump Inhibiting Peptide, SPIP. SPIP measured in the plasma of rats in hemorrhagic shock has been found in apparent concentration to be sufficient to inhibit completely membrane Na/K ATPase (compared to ouabain). The overall hypothesis is that hemorrhage elevates plasma SPIP which, in turn, inhibits membrane Na/K ATPase producing cell depolarization and increased cell sodium and water, contributing to the development of hemorrhagic shock. The membrane Ca/Na ATPase exchanger, then pumps Na out and Ca in, elevating intracellular Ca. In this proposal, the applicant will 1) determine if resuscitation will prevent or attenuate the rise in plasma SPIP after hemorrhage, 2) determine if SPIP is secreted by cultured endothelial cells in response to hormones known to be elevated after hemorrhage and known to induce PAI-1 mRNA and/or protein, 3) determine if SPIP-mediated inhibition of the Na/K ATPase causes intracellular accumulation of Ca++, 4) determine if hemorrhage elevates mediators in plasma that stimulate secretion of SPIP into plasma, and 5) identify the organs and cell types that show a hemorrhage-induced rise in PAI-1 mRNA, the precursor to SPIP.